Backscatter communication systems are known in the art. In a backscatter system, one transponder, such as an interrogator, sends out a command to a remote communications device. After the interrogator transmits the command, and is expecting a response, the interrogator switches to a CW mode (continuous wave mode). In the continuous wave mode; the interrogator does not transmit any information. Instead, the interrogator just transmits radiation at a certain frequency. In other words, the signal transmitted by the interrogator is not modulated. After a remote communications device receives a command from the interrogator, the remote communications device processes the command. The remote communications device of the backscatter system modulates the continuous wave by switching between absorbing RF radiation and reflecting RF radiation. For example, the remote communications device alternately reflects or does not reflect the signal from the interrogator to send its reply. Two halves of a dipole antenna can be either shorted together or isolated from each other to modulate the continuous wave.
One example of a backscatter system is described in commonly assigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, and incorporated herein by reference. Another example of a backscatter system is described in U.S. Pat. No. 5,649,296 to MacLellan et al. which is also incorporated herein by reference.
One application for backscatter communications is in wireless electronic identification systems, such as those including radio frequency identification devices. Of course, other applications for backscatter communications exist as well. Most presently available radio frequency identification devices utilize a magnetic coupling system. An identification device is usually provided with a unique identification code in order to distinguish between a number of different devices. Typically, the devices are entirely passive (have no power supply), which results in a small and portable package. However, such identification systems are only capable of operation over a relatively short range, limited by the size of a magnetic field used to supply power to the devices and to communicate with the devices.
Another wireless electronic identification system utilizes a large, board level, active transponder device affixed to an object to be monitored which receives a signal from an interrogator. The device receives the signal, then generates and transmits a responsive signal. The interrogation signal and the responsive signal are typically radio-frequency (RF) signals produced by an RF transmitter circuit. Because active devices have their own power sources. The active devices do not need to be in close proximity to an interrogator or reader to receive power via magnetic coupling. Therefore, active transponder devices tend to be more suitable for applications requiring tracking of objects that may not be in close proximity to the interrogator, such as a railway car.
Spread spectrum modulation techniques are known Utilization of spread spectrum modulation provides distinct advantages in some communication applications. For example, some spread spectrum modulation techniques enable desired signals to be distinguished from other signals (e.g., radar, microwave ovens, etc.) operating at approximately the same frequencies.
Federal Communication Commission (FCC) regulations require that spread spectrum systems meet various requirements. For example, spread spectrum systems operating in the 2.4-2.485 GHz band should comply with FCC rule 15.247 which states, in relevant part, that the power spectral density cannot be more than +8 dBm in any given 3 kHz bang. Further, the maximum power output is 1 Watt into a 6 dBi gain antenna. The minimum 6 dB bandwidth for a direct sequence spread spectrum is 500 kHz. In addition, the energy within restricted bands of 0-2.390 GHz and 2.4835-2.5 GHz should be lower than 500 uV/m at three meters. Communication systems operating within this specified band should be designed with regard to these regulations.
Amplitude modulation (AM) communication techniques enable communications with the use of relatively straightforward detectors. Typically, such AM detectors can be efficiently implemented with the utilization of relatively few components. However, drawbacks exist with the utilization of amplitude modulation techniques. For example, approximately half the total power of AM communications resides' within the carrier. This limits the power which can be used for communicating data if AM modulation and spread spectrum techniques are utilized within the above specified frequency band.
Therefore, there exists a need to provide communication systems which comply with radio frequency regulations while also providing robust communications.